1. Field of the Invention
The present invention relates to a joint structure for a robot, and more particularly to a joint structure for a robot which includes a motor and a speed reducer.
2. Description of the Related Art
A multi-joint manipulator is known as an industrial robot. The robot includes multiple joints such as a shoulder joint or an elbow joint, and at each joint, a first member and a second member are coupled to each other via a rotational driving mechanism. For the rotational driving mechanism, use is made of a motor, a speed reducer, gears, and so forth.
FIG. 4 is a sectional view schematically illustrating a prior art joint structure for a robot. Referring to FIG. 4, a second member 3 is coupled to a first member 1 via a speed reducer 2. For the speed reducer 2, use is made of a planetary gear speed reducer, a wave gear device or the like. Such speed reducer 2 is configured of an output unit, an input unit, and a fixing unit, but in FIG. 4, each form of the output unit, the input unit, and the fixing unit of the speed reducer 2 is omitted.
As illustrated in FIG. 4, the speed reducer 2 is coupled to one surface of a wall portion 1b of the first member 1 by bolts 6, and a motor 4 is located in a space 1a at a side opposite to the one surface. A first gear 5 is attached to a shaft of the motor 4. Further, the second member 3 is coupled to the output unit of the speed reducer 2. A second gear 7 is attached to a shaft of the input unit of the speed reducer 2.
Further, the wall portion 1b of the first member 1 is formed with a recess 8 that accommodates the second gear 7 of the speed reducer 2. The recess 8 is formed by recessing the wall portion 1b. An opening 8a of the recess 8 is formed so as to allow the second gear 7 to pass through. Further, on the bottom of the recess 8 of the first member 1, a communication hole 9 is formed that allows the interior of the recess 8 to be communicated with the space 1a. The shaft of the motor 4 is passed through the communication hole 9, and the motor 4 is supported by and fixed to a periphery of the communication hole 9 so as to close the communication hole 9.
As illustrated in FIG. 4, when the speed reducer 2 is coupled to the wall portion 1b of the first member 1, the second gear 7 is disposed in the recess 8 of the first member 1 so as to intermesh with the first gear 5. The rotational driving force of the motor 4 is amplified by the first gear 5 and the second gear 7 and then inputted to the speed reducer 2, and the inputted rotational driving force is further amplified in the interior of the speed reducer 2, whereby the second member 3 is swung about a predetermined axis of rotation.
Further, with the above-described structure, when the speed reducer 2 is coupled to the wall portion 1b of the first member 1 so as to locate the second gear 7 in the recess 8, the speed reducer 2 closes the opening 8a of the recess 8. Hence, before the speed reducer 2 is coupled to the first member 1, it is preferably that the second gear 7 be preattached to the input unit of the speed reducer 2. Thus, may the second gear 7 be changed to a gear having a larger diameter in order to increase the speed reduction ratio, a problem would occur in which the second gear 7 attached to the speed reducer 2 interferes with the wall portion 1b of the first member 1 when the speed reducer 2 is coupled to the first member 1. Further, in order to prevent the problem, if the opening 8a of the recess 8 in the first member 1 is made larger than the diameter D of a part 2a (hereinafter, referred to as coupling unit) of the speed reducer 2 which allows the speed reducer to be coupled to the first member 1, it would not possible to couple the speed reducer 2 to the first member 1. Therefore, when the speed reduction ratio is changed, it is not possible to make the outer diameter of the second gear 7 larger than the diameter D of the coupling unit 2a of the speed reducer 2.
As the result, in the structure illustrated in FIG. 4, there is a limit for setting a larger speed reduction ratio by increasing the gear diameter of the input unit of the speed reducer 2 for the purpose of increasing the torque for driving the second member 3.
In contrast, Japanese Laid-open Patent Publication No. H08-155881 discloses a structure in which a speed reducer is coupled to an arm portion in which a motor is accommodated, wherein a wall portion of the arm portion to which the speed reducer is coupled is configured to be splittable so as to make it possible to use a gear having a larger diameter than the body of the speed reducer. FIG. 5 is a schematic sectional view of the structure. In FIG. 5, components similar to those illustrated in FIG. 4 are denoted by the same reference numerals.
In the structure illustrated in FIG. 5, a coupling plate 10 for coupling a first member 1 and a speed reducer 2 to each other is located between the first member 1 and the speed reducer 2. The coupling plate 10 is removably fixed to a wall portion 1b of the first member 1 by bolts 6. Further, the speed reducer 2 is removably fixed by bolts 11 to the coupling plate 10 fixed to the wall portion 1b of the first member 1.
Further, in the structure illustrated in FIG. 5, a driving force for moving the second member 3 with the first member 1 as a datum passes through a power transmission path 12 such as indicated by a broken line arrow in FIG. 5. In other words, the driving force generated from the motor 4 and the gears 5 and 7 is transmitted from the first member 1 to the second member 3 successively via the bolts 6, the coupling plate 10, the bolts 11, and the speed reducer 2. Thus, in the case of the structure illustrated in FIG. 5, bolts capable of withstanding power transmission are used for the bolts 6 and 11.
According to the structure illustrated in FIG. 5, the first member 1 and the speed reducer 2 are coupled to each other via the coupling plate 10, and thus the opening 8a of the recess 8 of the first member 1 can be widened to be larger than the diameter D of the coupling unit 2a of the speed reducer 2. In this manner, it is possible to increase the outer diameter of the second gear 7 to be larger than the diameter D of the coupling unit 2a of the speed reducer 2.
However, in the structure illustrated in FIG. 5, the power transmission path 12 between the first member 1 and the speed reducer 2 is cut by the coupling plate 10 which couples the first member 1 and the speed reducer 2 to each other. Thus, not only the bolts 6 and 11 but also the coupling plate 10 is preferred to have a strength capable of withstanding power transmission. As a result, a problem arises in which the component cost and the number of man-hours are increased.
Further, without making the diameter of the second gear 7 smaller than the part to which the coupling plate 10 of the first member 1 is coupled, i.e., the periphery of the opening 8a of the recess 8 as illustrated in FIG. 5, it is not possible to couple the plate 10 to the periphery. As such, in the case of the structure illustrated in FIG. 5, a new problem arises in which while the diameter of the second gear 7 can be increased to be larger than the diameter D of the coupling unit 2a of the speed reducer 2, the increase in the gear diameter is limited by the size of the opening 8a of the recess 8 in the first member 1
Further, there is also a problem in which the second gear 7 may not be easily exchanged since it is preferable to remove from the first member 1 the relatively heavy speed reducer and the second member 3 attached to the speed reducer when the second gear 7 is exchanged for maintenance.